Opioid modulation of Fos protein expression and olfactory circuitry plays a pivotal role in what neonates remember

Abstract

Paradoxically, fear conditioning (odor-0.5 mA shock) yields a learned odor preference in the neonate, presumably due to a unique learning and memory circuit that does not include apparent amygdala participation. Post-training opioid antagonism with naltrexone (NTX) blocks consolidation of this odor preference and instead yields memory of a learned odor aversion. Here we characterize the neural circuitry underlying this switch during memory consolidation. Experiment 1 assessed post-training opioid modulation of Fos protein expression within olfactory circuitry (olfactory bulb, piriform cortex, amygdala). Odor-shock conditioning with no post-training treatment (odor preference) induced significant changes in Fos protein expression in the granule cell layer of the olfactory bulb and anterior piriform cortex. Post-training opioid receptor antagonism (odor aversion) prevented the learning-induced changes in the anterior piriform cortex and also induced significant changes in Fos protein expression in the central nucleus of the amygdala. Experiment 2 assessed intra-amygdala opioid modulation of neonate memory consolidation. Post-training infusion of NTX within the amygdala permitted consolidation of an odor aversion, while vehicle-infused pups continued to demonstrate an odor preference. Overall, results demonstrate that opioids modulate memory consolidation in the neonate via modulating Fos protein expression in olfactory circuitry. Furthermore, these results suggest that opioids are instrumental in suppressing neonate fear behavior via modulating the amygdala.Odor-shock conditioning, a learning paradigm that readily produces fear in adult animals, produces a paradoxical odor preference in rat pups <10 d old (Camp and Rudy 1988;Sullivan et al. 2000a;Roth and Sullivan 2001, 2003Moriceau and Sullivan 2004a). This paradoxical behavior is attributable to a relatively simplistic olfactory learning and memory circuit that incorporates the olfactory bulb, anterior piriform cortex, and an apparent lack of amygdala participation (Sullivan et al. 2000a,b; Moriceau and Sullivan 2004a,b;Roth and Sullivan 2005;Moriceau et al. 2006). Indeed, we have demonstrated that the emergence of learned fear coincides with significant amygdala participation (Sullivan et al. 2000a;Moriceau et al. 2006). We suggest this attenuated aversion learning and potentiated preference learning early in development ensures pups readily learn to approach the maternal odor regardless of the quality of care she provides. Indeed, a recent study from our laboratory has documented this paradoxical odor preference learning within a seminatural nest environment as an odor paired with rough handling from a stressed mother resulted in a preference for that odor (Roth and Sullivan 2005). A similar paradoxical attachment despite caregiver abuse has been demonstrated in other species: chick (Hess 1962), infant dogs (Rajecki et al. 1978), and nonhuman primates (Harlow and Harlow 1965;Maestripieri et al. 1997Maestripieri et al. , 1999.We have previously demonstrated a ...

Ontogeny of odor-LiCl vs. odor-shock learning: Similar behaviors but divergent ages of functional amygdala emergence

“…Previous data on infant odor learning indicate that the olfactory bulb exhibits robust learning-associated changes in early life, regardless of pups learning an odor preference or odor aversion (Sullivan and Leon 1986;Wilson et al 1987;Sullivan et al 1990;Woo et al 1996;McLean et al 1999;Yuan et al 2003;Roth et al 2006). The role of the olfactory bulb changes with development since older pups and adults do not usually show odor learning-induced changes in this brain area (Hamrick et al 1993;Woo et al 1996;Sevelinges et al 2007).…”

“…As illustrated in Figure 2 , LiCl and 1.2-mA shock resulted in pups of all ages learning an odor aversion, while 0.5-mA shock switched from producing an odor preference in younger pups to an aversion in older pups (Garcia et al 1966(Garcia et al , 1974Hennessey et al 1976;Haroutunian and Campbell 1979;Smotherman 1982;Stickrod et al 1982;Rudy and Cheatle 1983;Kucharski and Spear 1984;Smotherman andRobinson 1985, 1990;Alleva and Calamandrei 1986;Miller et al 1990b;Best 1992, 1993;Sullivan and Wilson 1995;Sullivan et al 2000a;Roth and Sullivan 2001;Richardson and McNally 2003;Gruest et al 2004;Moriceau and Sullivan 2004;Roth et al 2006;Shionoya et al 2006). …”

“…In contrast to adult odor-LiCl learning, which relies on the amygdala (Touzani and Sclafani 2005), this early-life, odoraversion learning relies on the olfactory bulb until the pup approaches weaning age, when the amygdala is incorporated into the learning circuitry (Shionoya et al 2006). In contrast, if infant rats receive an odor paired with a moderately painful stimulus (0.5-mA foot or tail shock, or tail pinch) the amygdala appears to be incorporated into this learning circuitry around postnatal day Here we expand assessment of the developing pups' odoraversion learning circuit by including the anterior and posterior piriform cortex, which have previously been demonstrated to be important for both pup and adult odor learning (Litaudon et al 1997;Barkai and Saar 2001;Mouly et al 2001;Mouly and Gervais 2002;Tronel and Sara 2002;Moriceau and Sullivan 2004;Sevelinges et al 2004;Wilson et al 2004;Roth et al 2006). We also extend the assessment of the development of odor-aversion learning by directly comparing a range of odor-aversion learning paradigms and including different intensities of shock.…”

“…Here we expand assessment of the developing pups' odoraversion learning circuit by including the anterior and posterior piriform cortex, which have previously been demonstrated to be important for both pup and adult odor learning (Litaudon et al 1997;Barkai and Saar 2001;Mouly et al 2001;Mouly and Gervais 2002;Tronel and Sara 2002;Moriceau and Sullivan 2004;Sevelinges et al 2004;Wilson et al 2004;Roth et al 2006). We also extend the assessment of the development of odor-aversion learning by directly comparing a range of odor-aversion learning paradigms and including different intensities of shock.…”

“…We used rats ranging from an age of complete dependence on the mother through an age of independence at weaning. We focused on the amygdala basolateral complex, which is implicated in adult fear conditioning and odor-malaise learning (Bermudez-Rattoni et al 1986;LeDoux 2000;Batsell and Blankenship 2002;Gale et al 2004;Holland and Gallagher 2004), and the anterior and posterior piriform cortex, olfactory pathway areas that have previously been implicated in infant odor-aversion and odor-preference learning Roth and Sullivan 2005;Roth et al 2006) as well as adult odor learning (Ferry and Di Scala 1997;Litaudon et al 1997;Barkai and Saar 2001;Mouly et al 2001;Mouly and Gervais 2002;Tronel and Sara 2002;Sevelinges et al 2004;Wilson et al 2004;Touzani and Sclafani 2005).…”